The provision of an audio signal, especially music, in a digital form is becoming an increasingly popular method of distributing digital data. Increasingly, however, as music and other audio data is increasingly being distributed by downloads and file sharing, rather than being provided in a hardware form such as a compact disc, the ease with which data can be copied makes commercial applications of digital music distribution problematic.
There is a need for example for copyright material to be embedded in a digital data file in such a manner that it can be determined whether or not a digital audio data file is an authorized file, or an illegal copy. To ensure the integrity of such a system, the copyright material needs to be secure and free from tampering. Similarly it would be desirable for commercial reasons to be able to introduce advertising material into a digital audio file, but such advertising material also needs to be secure if it is to be effective. The purpose of including the advertising material would be lost if it could easily be removed from the digital audio data file. It would also be commercially useful to be able to provide prospective purchasers of a trial version of a digital audio data file prior to purchase, but if the trial version can be copied and circulated itself, then this would be counter-productive.